High power acoustic radiator



Juhe 23, 1970 WHITEHEAD 3,517,390

HIGH POWER ACOUSTIC RADIATOR Original Filed Oct. 14, 1965 2 Sheets-Sheet1 go m 34 52 DRIVER 20 i l m l DRIVER DFQ/VER I FIG. I B

DR/VER 4g DRIVER Layhe Whitehead INVENTOR BY Michael F. Breston ATTORNEYDR/ VE R FIG. 4

June 23, 1970 WHITEHEAD "3,517,390

HIGH POWER ACOUSTIC RADIATOR Original Filed Oct. 14, 1965 z Sheets-Sheet2 DRIVER LOAD F G. 6 f0 FREQ. I

Layne Whitehead F G. 5

IN VE N TOR BY Michael PBreston ATTORNEY United States Patent 3,517,390HIGH POWER ACOUSTIC RADIATOR Layne Whitehead, 7220 Selma St., Houston,Tex. 77025 Continuation of application Ser. No. 496,746, Oct. 14, 1965.This application Feb. 29, 1968, Ser. No. 709,388 Int. Cl. G08b 3/00; Gk9/00 US. Cl. 340-384 17 Claims ABSTRACT OF THE DISCLOSURE This inventionrelates to a new and improved acoustic radiator such as a fog radiatorfor allowing an acoustic source to generate a relatively large amount ofacoustic power into a gas medium. During each cycle, a portion of theacoustic energy from the source is stored in the radiator to establishstanding pressure waves therein; the remainder of the energy is radiatedto the gas medium. The geometry of the radiator is selected so that thevibrating gases confined therein present an optimum load to the acousticsource thereby allowing it to generate optimum power into the gasmedium. The radiator can be made in sections having diversecross-sectional areas.

This application is a continuation of applicants copending applicationSer. No. 496,746, now abandoned, entitled Acoustical Impedance Matchingand Array Forming.

BACKGROUND OF THE INVENTION While the teaching of this invention mayfind application in various fields, it will be described as beingparticularly useful to the field of fog warning devices such as arerequired by the US. Coast Guard for offshore platforms. Warning devicesfor emitting audible long range signals to warn ships of dangerousobstacles or to signal them are well known. In the recent past, offshoreoil production and oceanographic work have greatly expanded. The needfor dependable, eflicient and long-lasting acoustic radiators is evergrowing.

It was generally accepted by workers with fog warning devices that theclassic design theory of horns provided optimum design parameters.Although various geometric horn configurations were built, the classicdesign principles of horns were strictly adhered to. These principleswere based on the following factors: When a diaphragm is set in motionin free space the air particles in front of it are given a certainvelocity. The pressure of the diaphragm against the volume of air infront of it is accompanied by a reaction pressure against the diaphragm.This reaction pressure is proportional to the air particle-velocity, andmust obviously be small compared with the forces due to the inertia andstiffness of the diaphragm itself. The diaphragm therefore works into avery small load, and its motion is almost entirely determined by its ownstiffness and mass. Consequently the useful work done by the diaphragmon the air will in general be very small.

The paramount function of the horn is to properly load the diaphragm. Anacoustic generator without a load is like an engine without a load orlike a radio transmitter without an antenna.

In addition to providing a load to the diaphragm the horn is designed tomeet three important requirements: (1) A given applied force acting onthe diaphragm must cause the air at the throat of the horn to have anearly uniform air volume velocity at the operating frequency; (2) thearea of the mouth of the horn must be such that little sound energy isreflected, otherwise air-column resonances will occur; and (3) the lawof increase of area of cross section with length and the rate of areaincrease must be such as to avoid discontinuities, that is, standing3,517,390 Patented June 23, 1970 waves. This will provide a constantratio of pressure to air volume velocity at the throat at the operatingfrequency. That ratio is known as the throat impedance.

If the preceding requirements are met and if the throat area is smallcompared with the diaphragm area, the air in the throat is given aproportionately high particle velocity. As the pressure generated isproportional to this velocity, the reaction pressure on the diaphragm iscorrespondingly high and the work done on the air correspondingly great.It appears that the smaller the area of the throat, the more eflicientthe horn becomes. If the diameter of the throat is less than a certainvalue, however, the energy used up in overcoming viscous resistancebecomes intolerably high. Also, the smaller the throat the longer thehorn must be, and this is an additional disadvantage.

Another limiting factor is found in the requirement on the area of themouth of the horn. This area must be large enough to give negligiblereflection at the operating frequency. Since a horn is designed toeliminate reflection waves from its open end, its mouth is made as wideas possible; the wider the mouth the less reflection there is from theopen end. In sum, properly designed horns do not allow standing waveswithin their confining walls.

To serve as a fog warning device it is desirable that the horn, for agiven quantity of input energy, radiate a maximum amount of audio energyin the horizontal direction. This can be best accomplished by using anarray of horns. Since the designer was already committed to a smallthroat and a large mouth he could not escape from using a relativelylong horn which was frequently folded. An array of folded hornsconstitutes a heavy and bulky structure. One such structure is shownapplicants Pat. 3,153,783.

SUMMARY OF THE INVENTION The paramount function of the acoustic radiatorof this invention is much like that of the horn that is to load thediaphragm.

This radiator, however, is based on entirely different designconsiderations. The area of each mouth of the radiator is selected sothat appreciable sound energy is reflected back from the mouth toestablish air-column resonances, that is, standing waves within theconfining walls of the radiator which can be made in sections.

The law of increase of area of cross section with length is particularlyadapted to benefit from sharp changes in the cross sectional areasbetween consecutive sections forming the radiator. Thus, the radiator ofthis invention, instead of avoiding, suppressing, and minimizingstanding waves or air column resonances within its confining walls,makes good use of them. The air columns within the radiator aresubjected to forced vibrations. If the frequencies of the vibratingdiaphragm coincide with the natural frequencies of the radiator having aproper acoustic length, the amplitude of the reaction pressure againstthe diaphragm may become very large indeed. During part of the vibrationcycle of the diaphragm, the radiator is drawing energy from thediaphragm. An appreciable portion of that energy is stored in the aircolumn confined between the walls of the radiator in the form ofstanding waves. Some Waves escape from the open end of the radiator todo work on the surrounding air. The radiator therefore makes use ofbackward-and-forward traveling waves to establish a vibrating soundfield within the in terior of the radiator. The outward-and-backwardtraveling waves add in magnitude to produce standing waves which allowpower to flow toward the open end of the radiator. While the sectionsforming the radiator of this invention can assume variousconfigurations, in practice it is convenient to Work withregularly-shaped sections having circular or rectangular cross-sectionalareas. Cylindrical sections in particular lend themselves better toanalytical treatment.

Analytically and experimentally, the teachings of this invention allowthe building of simple and complex acoustic radiators. The buildingblocks for these radiators are sections in which air-columns resonate(called resonators).

The teachings of the present invention avoid the undesirable designlimitations previously discussed in connection with fog-horns and allowthe design of eflicient, relatively-inexpensive, compact radiators whichare particularly useful as fog warning devices for offshore platforms.

It is a primary object of the present invention to provide an improvedacoustic radiator capable of efficient operation with conventionalsignal generating equipment.

It is another object of the present invention to provide a improvedacoustic radiator, acting as an impedance transformer in conjunctionwith a conventional acoustic generator to provide efficiet and optimumoperation.

Another object of the present invention is to provide such an acousticradiator which can be built in sections to form arrays.

A further object of the present invention is to provide such an acousticradiator which is relatively light, occupies a minimum of volume, and isrelatively simple to manufacture.

The novel features which are believed to be characteristic of theinvention, both as to its construction and method of operation, togetherwith further objects and advantages thereof will be better understoodfrom the following description considered in connection with theaccompanying drawings in which presently preferred embodiments of theinvention are illustrated by way of example. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and descrip tion only, and are not intended as a definitionof the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the drawings:

FIG. 1A shows in cross-section a driver coupled to a single resonantcoupler;

FIG. 1B shows an array of three couplers.

FIG. 2 shows schematically a driver feeding four single-step resonantcouplers.

FIG. 3 shows another arrangement of a driver feeding three single-stepresonant couplers;

FIG. 4 shows schematically a driver feeding three couplers each ofincreasing cross-sectional area;

FIG. 5 shows various response curves;

FIG. 6 shows a broadside array of multi-step resonant couplers; and

FIG. 7 shows another arrangement of a broadside array of multi-stepresonant couplers.

Although the present invention is described hereinafter in connectionwith fog warning devices and particularly with respect to the generationof cyclic signals of narrowband frequency it is to be understood thatthe present invention is equally applicable to the generation ofacoustic signals for other purposes.

Referring to FIGS. 1A and 1B there is shown a fog radiator generallydesignated as 10 having a support structure 12 for supporting threeresonators 14, 16 and 18, driven by untuned electro-magnetic drivers 20,22 and 24, respectively. Each driver has a permanent magnetic structure25, a coil 26 and a diaphragm 28. Drivers of this type are commerciallyavailable and are used primarily for public address systems. Analternating-current electric signal is applied. to the input terminals30, 32 of each coil 26. The signal may have a square shape. Preferablyit is sinusoidal. Each resonator consists of a section of cylindricalpipe having a radius R and a length L. The drivers and resonators aresuitably secured to ribs 13 of the structure 12 as by welding. Thelength L is selected so that the fundamental resonant frequency of theair column confined within the Walls of the tube resonates at afundamental frequency corresponding to the frequency 4 f of the signalapplied to coil 26. A standing wave is then established within each ofresonators 14, 1'6 and 18 having a wavelength \=c/f, where c is thevelocity of sound in air. The length L is chosen so that the effectiveacoustic length of each tube is one quarter wavelength or an oddmultiple thereof, i.e.

where n is an integer. Each driver-tube combination can be thought of asa coupled system consisting of a driver and a vibrating air column. Thecolumn is the predominant element in this combination and optimumresults are achieved when the frequency of the vibrating diaphragmcoincides with the frequency of the fundamental mode of vibration of theair column. As is known to those skilled in the art, the effectiveacoustical length of each pipe is not equal to L because of what isknown as the end-effect. For a cylindrical pipe the end correction isabout 0.6R. The magnitude of the end correction for other thancylindrical pipes depends on the degree of openness to the air. Thus fora tube the actual value of L is One can think of the maintenance of airvibrations in each tube as follows: As the diaphragm vibrates back andforth it produces sound waves which travel into the tube. The intialvibration of the diaphragm causes a compression to enter into the pipe.The compression travels up to the open end of the pipe. A small fractionof the energy in the compression is radiated into the open air; theremainder is reflected back toward the diaphragm. When the reflectedwave reaches the diaphragm, it has traveled one-half Wavelength sincethe effective acoustical length of the tube is one-quarter wavelength.At this instant, the diaphragm reverses its direction of travel, hencethe reflected wave is in phase with the wave produced by the diaphragm.Since the forward traveling wave is in phase with the backward travelingwave, the two waves add and produce what is known as a standing Wave inthe tube. The standing wave has a maximum air volume velocity and aminimum pressure at the open end or mouth of the tube, and a minimumvolume velocity and a maximum pressure at the throat of the tube. Thismaximum pressure at the throat exerts a great reaction pressure againstthe diaphragm which allows the diaphragm to pump a correspondingly greatquantity of energy into the tube. In other words, as the driver feedspower into the tube against a maximum reaction pressure, it sees a muchhigher load impedance than it would see if the driver worked directlyinto the air. It is known that for maximum efliciency the driver shouldwork into a specific load. It is possible to provide this specific loadby suitably selecting the dimensions of the tube. It may also be helpfulto think of the tube as an impedance transformer transforming the lowimpedance at the mouth of the tube into a. high impedance at the throatof the tube.

While resonators 14', 16 and 18 have been described in connection with acylindrical tube having a circular cross section for the sake ofsimplicity, it will be understood by those skilled in the art that theresonators may have a. rectangular cross section or any other crosssectional area. It is desired to maintain standing waves only along thelongitudinal axis, hence the diameter of the tube should besubstantially less than a wavelength corresponding to the fundamentalfrequency of the driver.

For a fog radiator it is desirable to have an array of sound sourcesarranged to make the sound intensity greater in a given directionusually horizontal. In FIG. 1B resonators 14, 16 and 18 are arranged inwhat is known as a broadside or linear array. The spacings between theopen ends of the resonators should be properly adjusted for the desireddirectivity pattern. For a full discussion of directivity patterns anddesirable spacings reference is made to a text book on Acoustics by LeoL. Beranek, published by McGraw-Hill, 1954.

In FIG. 1B, the drivers are mounted on top of the resonators and arecovered with a cover 34 to protect them from rain and snow. It isdesirable to drive all the drivers with electric signals having the samefrequency and phase. If there is a phase difference between any twoelectric signals, undesirable pressure cancellations may take place inthe open air. All the drivers may be driven from the same electricsource. If necessary, phase shift networks may be used in the circuitsto compensate for phase differences.

When viewed from the driver, each resonator in FIG.

1B represents a single-step resonator, i.e., the driver sees only oneimpedance changing load sometimes referred to as coupler. In FIG. 2 isshown a radiator 40 having four singlestep resonators 42, 43, 44 and 45connected in parallel to a single driver 46. The single-step resonators42, 43 may each have an effective acoustic length of A or /1 wavelengthat the drivers operating frequency. Resonators 414, 445 may each be /4)\or 7/4 long. The ends of resonators 44, 45 are curved to fit therequirements of array forming. The space separations between the openends of the resonators 42-45 should be suitably selected for the desireddirectivity pattern. Since the theory of operation of odd-multiplequarter wavelength couplers is the same as that of single quarterwavelength couplers, reference is made to the prior description.

In FIG. 3 is shown a different arrangement of a radiator 50 having threesingle-step resonators 52, 53 and 54 connected in parallel and driven bya single driver 56 similar to that shown in FIG. 2. Each resonator has alength equivalent to an odd multiple quarter wavelength at the operatingfrequency of the driver. The open ends of the resonators are bent tokeep water and snow out as Well as to space them to form a linear array.

So far the arrays were shown as being formed of singlestep resonators.Such arrays are susceptible of having the effective acoustic length ofeach coupler or resonator change with changes in environmentalconditions, primarily temperature. When the air temperature changes, thesound velocity c in the air also changes and hence A changes, therebyintroducing a phase shift between the forward and backward travelingwaves. That phase shift can deleteriously affect the load of the aircolumn precented to the diaphragm. One way to overcome the consequencesof changes in the wavelength x is to use a driver, the frequency ofwhich is automatically regulated with changes in temperature. Forexample, the oscillator generating the electric signal for the drivermay have a frequency-determining component (a resistor or capacitor)therein, having a value which changes with temperature by an amountsufficient to offset the change in the wavelength with temperature, andhence in the acoustic length of the resonator. Another method is to usea feedback loop including the driver or a microphone in the coupler tokeep the operating frequency at an optimum value.

In sum, without compensating means, single-step couplers have a verynarrow frequency pass-band. In practice, this means that due to a changein temperature, the acoustic length of the radiator may becomeineffective to properly load the diaphragm and, therefore, to accomplishuseful work on the outside air. Of course, if compensating or feed-backmeans are provided than any change in the effective acoustic length ofthe radiator will automatically be compensated by a corresponding changein the frequency of the operating electric signal, and no net change inthe radiated acoustic power will be noted. In accordance with one aspectof this invention, there is provided a radiator including a cascade ofsuitably connected resonators which allows; (1) to broaden the frequencypass-band of the radiator, and (2) to achieve a greater impedancetransformation ratio.

In FIG. 4 is shown a cascade or series assembly 65 of threequarter-wavelength sections 61, 62 and 63 driven by a driver 60. For anunderstanding of the operation of a cascaded radiator, it may be helpfulto remember that coupler 63 constitutes the load for coupler 62, and thelatter forms the load for coupler 61. The interface between thefirst-step coupler 61 and the second-step coupler 62 marks a substantialchange (step) from the cross sectional area of the first coupler to thesecond coupler. A step is also provided between the second and thirdcouplers 62 and 63. The ratio between adjacent couplers areas isselected to give an impedance mismatch between the couplers. Dependingupon the amount of impedance mismatch, reflections will occur at theinterfaces, thereby creating a sound field to whereinforward-and-backward traveling waves add to 'form standing waves in eachcoupler. In general, a coupler presents a resistive load and a reactiveload to an adjacent coupler. At resonance, a quarter-wavelength couplerpresents primarily a resistive load: the reactive load substantiallyvanishes. If the coupler is shorter than a quarter wavelength, then itpresents a negative reactive load; if it is longer than aquarterweavelength it presents a positive reactive load. These reactiveloads are vector quantities. In a quarter-wavelength coupler thepositive and negative reactive loads are equal in magnitude, and hencetheir resultant is zero. Each coupler may be represented by anequivalent electrical circuit using resistive, capacitive, and inductiveele ments. Thus, for example, a quarter-wavelength coupler may berepresented by a series or parallel tuned circuit where the inductiveimpedance is equal to the capacitive impedance. Whereas in an electricalresonant circuit there is a cyclic transfer of energy between theinductive and capacitive loads, in a quarter-Wavelength resonator thereis a cyclic transfer between the kinetic and potential energles storedin the air column confined between the walls of the coupler. In thisrespect, the air column is similar to a pendulum, a tuning fork, or astretched elastic string. Couplers shorter or greater than aquarter-wavelength may be represented by equivalent resistive andcapacitive or inductive loads. On the other hand, the radiated powerfrom the open end of the radiator constitutes a resistive loaddesignated hereinafter as R In the cascaded radiators just as in thesingle step radiators, it 1s desired to transform R into a much greaterload nR where n is the transformation ratio of the radiator.

The value of the transformation ratio n of an cascaded radiator assemblydepends on the transformation ratio of each coupler, in turn, depends onits pressure standing Wave ratio which can be calculated from aknowledge of the geometric parameters, or it can be experimentallymeasured. The pressure within a coupler can be easily measured byinserting a small hollow probe tube into the coupler. The ratio betweenthe maximum pressure and the minimum pressure represents the pressurestanding wave ratio. If the pressure is uniform througout a coupler, thestanding pressure wave ratio is one: no standing gvlalves exist and thecoupler acts as a conductor of sound In sum, the load of each couplerdepends on its pres sure standing wave ratio, which in turn depends onits cross-sectional area. The total transformation ratio n of theradiator assembly can therefore be analytically or experimentallydetermined from a knowledge of the individual couplers standing waveratios. For example, the standing wave ratio in a cylindrical coupleropening into the air can be approximately calculated as the ratio of thearea of the coupler to the area of a circle whose diameter is twice thewavelength, at the operating frequency a divided by 1r.

Because acoustical phenomena are continuous, slight deviations fromideal design parameters produce only slight deviations from desiredresults. This allows multiple step resonators to be built from standardsize pipes.

It will be appreciated that the reactive impedance of a coupler dependson the deviation of the Wavelength from the design wavelength. Aspreviously mentioned, a change in temperature results in a correspondingchange in the wavelength. Each coupler when loaded with a reactive loadinverts its load about its characteristic impedance at its input end.Since the coupler also develops a reactive component due to the changein wavelength, the inverted component is partially cancelled by thedeveloped reactive component. The greater the number of couplers, theeasier it is to balance out the reactive components due to a change inthe wavelength of the radiated sound waves, i.e., to maintain the totaltransformation ratio n of the radiator constant. Consequently, amulti-step radiator allows a relatively Wider frequency pass-band.

Mathematically, it can be shown that for a desired particular typefrequency pass-band and transformation ratio, the ratios between thecross-sectional areas of the couplers can be made proportional to thecoefficients of certain mathematical functions. For example, the areasof the couplers can be proportional to the coeflicients of a binomialexpansion or of the well-known Tchebycheif polynomial. In the lattercase, the cascade of quarterwavelength resonators follows aTchebycheff-type frequency response. Because the computations becomesomewhat tedious they can be conveniently programmed on a digitalcomputer. The information derived can be tabulated to serve for thedesign of other cascaded step couplers.

In FIG. are shown three frequency response curves 70, 71 and 72.Frequency is plotted on the X axis and the acoustic response(normalized) on the Y axis. Curve 70 represents the response of asinglestep coupler. Curve 71 represents the response of a cascaded stepcoupler having a flat frequency response, and curve 72 represents theresponse of a cascaded step coupler having a rippled response or aTchebycheff-type response. Curve 70 has a relatively narrow frequencyresponse, that is, on either side of an operating center frequency i theresponse falls off sharply. Curve 71 represents a wider frequencyresponse, and within the pass-band the response is uniformly flat. Curve72 represents a rippled response within the pass-band. The amount ofripple that can be tolerated depends on the particular application. Bysuitably adjusting the size of the steps in a cascaded radiator, theamount of ripple can be controlled.

In FIG. 6 is shown a broadside array of couplers which are arranged in aplurality of assemblies acoustically connected in series-parallelcombinations. A driver 80 feeds into a coupler 81. Coupler 81 feedscouplers 82 and 83 connected in parallel. Since the load of a coupler isdetermined by its size or cross-sectional area rather than by its shape,the division of a main tube into two parallel branch tubes, each equalin size to the main tube, is equivalent to doubling the area of the maintube. As previously mentioned, if a tube adjoins a tube of greater orsmaller area, then a greater or smaller step results. Coupler 82 isconnected in series with a larger area coupler 84 which, in turn, isdivided into parallel couplers 85 and 86. Coupler 85 is connected inseries with an end coupler 87, and coupler 86 feeds into an end coupler88. Similarly, coupler 83 feeds into a larger size coupler 89 which, inturn, is divided into couplers 90 and 91. Coupler 90 feeds into an endcoupler 92 and coupler 91 feeds into an end coupler 93.

In a broadside array as shown in FIG. 6, the desired spacings betweenthe end couplers 87, 88, 92 and 93, to achieve a horizontal or otherdirectivity pattern, may not allow the couplers to be all one-quarterwavelength long. If one coupler is made shorter or longer thanone-quarter wavelength, then to offset its resulting reactive componentanother coupler in the array is made either shorter or longer thanone-quarter wavelength.

On the other hand, if it is necessary to make one or more couplers, forsound conduction purposes, several one-quarter-wavelength long, thepressure standing wave ratio in these couplers should be lower than Whatwould be used in a quarter wavelength coupler, so as to developapproximately the same reactive component as that oif a quarterwavelength coupler when operated of resonance. For example, if coupleris shorter than onequarter wavelength, then coupler 84 may be madeshorter than one-quarter wavelength to compensate for the reactivecomponent in coupler 85. Also, if couplers 82 and 83 are three-quarterwavelength long, then the standing wave ratio in these couplers shouldbe approximately onethird the optimum standing wave ratio forone-quarter wavelength coupler. The pressure standing wave ratio in acoupler may be made lower or higher by suitably selecting the area ofthe coupler. If standing waves are not desired in a particular coupler,then its size or area is selected to achieve that purpose. For example,if it were desired to have no standing waves in couplers 82 and 83 thenthe ratio of the cross-sectional areas of couplers 82 and 84 should beequal to the pressure standing wave ratio in coupler 84. It will beappreciated that the pres sure standing wave ratio in coupler 84 dependsonly on the load that coupler 84 is presented with, that is, on couplers85, 86, 87 and 88. The ratio between the area of a coupler and the areaof the load it is equivalent to is equal to the pressure standing waveratio in the coupler. If, for example, a tube having a four square inchcrosssectional area is found to have a pressure standing wave ratio offour-to-one, then it will load the acoustic source the same as aninfinite pipe of one square inch crosssectional area. Thus, the loadingeffect of that coupler is the same as that of an infinite pipe of onesquare inch area. The same holds whether the source of acoustic energyis a driver or a coupler.

The arrangement shown in FIG. 6 allows a relatively broad band-passwhich could be of the maximally flat type or of the rippled type, Thereason Why a relatively broad band-pass is desired is to allow forvariations in atmospheric conditions, which aflect the wavelength of thesound waves propagated in the air, and for variations in the operatingfrequency of the acoustic energy source. The manner in which the areasof the couplers, i.e., the steps, are selected determines the type offrequency passband obtained for a given number of couplers.

In FIG. 7 is shown another series-parallel arrangement of a multi-stepradiator which forms a broadside array.

A driver 101 feeds into a coupler 102. The output of coupler 102 is fedinto coupler 103. The output from coupler 103 is fed into couplers 104and 105. Coupler 104 feeds into end couplers 106 and 107. Coupler feedsinto end couplers 108 and 109. To preserve the crosssectional areas ofthe radiator, closed tubes 110, 111, and 112 are provided, as shown. Theflow of sound within the radiator from the driver is indicated by thearrows.

In FIG. 7 all tubes are concentric. The area of coupler 106 can beobtained from a knowledge of the inside diameter of tube 106 and theoutside diameter of tube 110. The other areas can be similarlycalculated or measured. The area changes and lengths in FIG. 7 aresimilar to the ones shown in FIG. 6 yet the physical configurations aredifferent. Other physical arrangements will readily suggest themselvesto men skilled in the art for the purpose of obtaining broadside arraysin accordance with this invention.

A reflection plate 115 is used to form an image of the end coupler 106,thereby making the effective acoustic length of the array longer thanits actual length. A similar reflection plate 116 is used to form animage of the end coupler 109. Another function of reflection plates 115and 116 is to allow the end couplers 106 and 109 to see the same load astheir symmetrically opposed couplers 107 and 108, respectively. In otherwords, coupler 107 sees coupler 108 and coupler 106 sees its reflectedimage. The same holds for couplers 108 and 109. The use of thereflection plates 115, 116 allows greater directivity or beaming, bothbecause the couplers are 9 evenly matched and because the effectivelength of the radiator is longer than the actual physical length.Reflection plates may also be used in the other embodiments, if desired.

Drivers of the electromagnetic type have both mass and compliance intheir moving parts, thereby leading to resonant effects. When suchdrivers are operated off resonance, the reactive component of thecurrent flowing through the voice coil resistance increases heat lossesand reduces the operating efficiency. The radiators of this inventionlend themselves to reducing the deleterious effects caused when thefrequency of the signal driving the driver shifts. Thus, the radiator isarranged to have a rcactance, as seen by the driver, equal to but of theopposite sign than the rcactance of the driver, when operated off thedesired operating frequency. For example, the first step in FIG. 6between couplers 81 and 82, 83 is chosen to be a two-to-one step whichis a larger step than would normally be indicated for the first step ofthe radiator. The relatively larger first step provides the reactivecomponent required to cancel or offset the reactive component of thedriver 80. Hence, by proper selection of the areas of the tubes both anoptimum load and driver rcactance cancellation may be achieved. In thismanner, the optimum driver efficiency within the pass-band is obtained.

As an example of a type of radiator shown in FIG. 6, which is driven bya 360 cycle signal, the following dimensions in inches are given for thesake of illustration only:

Tubes Length Diameter 87, 93 (each) While the invention has beendescribed in connection with particular type couplers, it is not limitedthereto and other embodiments will readily suggest themselves to thoseskilled in the art. Also, while electromagnetic drivers were discussedother type drivers can be equally employed.

What I claim is:

1. In combination,

a signal generator,

a sound-emitting structure for emitting powerful audible acousticsignals at a predetermined frequency into a fluid medium when saidstructure is coupled to the output of said signal generator, saidstructure comprising:

at least two distinct housings acoustically arranged in series, saidhousings having substantially different cross-sectional areas therebyestablishing a sound reflecting interface between said areas,

each housing defining at least one sound conducting chamber,

each chamber having a determined length dimension in the direction ofsound travel to cause each chamber to act as a resonator at saidpredetermined frequency when said structure is coupled to said signalgenerator,

said length dimension being related to (2nl))\/4 wherein n is an integerand A is the wave length of said acoustic signals, and

each chamber upon becoming a resonator having pressure standing wavesestablished therein to allow said generator to transmit into said mediumsaid powerful acoustic signals.

2. The combination as defined in claim 1 wherein said length dimensionis substantially equal to (2m1))\/4.

3. The combination as defined in claim 1 wherein said signal generatoris electrically driven.

4. The combination as defined in claim 1 wherein said fluid medium isair.

5. The combination as defined in claim 1 and further including:

at least one sound reflector positioned adjacent to said structure toallow said acoustic signals to become reflected from said reflector.

6. In combination,

a signal generator,

a sound-emitting structure for emitting powerful audible acousticsignals at a predetermined frequency into a fluid medium when saidstructure is coupled to the output of said signal generator, saidstructure comprising:

at least two assemblies acoustically connected in parallel,

each assembly including at least two distinct housings acousticallyarranged in series, said housings having substantially differentcross-sectional areas thereby establishing a sound reflecting interfacebetween said areas,

each housing defining at least one sound conducting chamber,

each chamber having a determined length dimension in the direction ofsound travel to cause each chamber to act as a resonator at saidpredetermined frequency when said structure is coupled to said signalgenerator,

said length dimension being related to (2n-1))\/4- wherein n is aninteger and A is the wave length of said acoustic signals, and

each chamber upon becoming a resonator having pressure standing wavesestablished therein to allow said generator to transmit into said mediumsaid powerful acoustic signals.

7. The combination as defined in claim 6 wherein said length dimensionis substantially equal to (2n-1))\/4.

8. The combination as defined in claim 6 wherein said signal generatoris electrically driven.

9. The combination as defined in claim 8 wherein said fluid medium isair.

10. The combination as defined in claim 9 wherein the chambers in saidstructure which transmit said acoustic signals into said medium aresuitably displaced from each other to allow said acoustic signals tobecome transmitted in a directional pattern.

11. The combination as defined in claim 10 and further including:

at least one sound reflector positioned adjacent to said structure toallow said acoustic signals to become reflected from said reflector.

12. In combination,

a signal generator,

a sound-emitting structure for emitting powerful audible acousticsignals at a predetermined frequency into a fluid medium when saidstructure is coupled to the output of said signal generator, saidstructure comprising:

a plurality of assemblies acoustically connected in series-parallelcombinations,

each assembly including at least two distinct housings acousticallyarranged in series, said housings having substantially differentcross-sectional areas thereby establishing a sound reflecting interfacebetween said areas,

each housing defining at least one sound conducting chamber,

each chamber having a determined length dimension in the direction ofsound travel to cause each chamber to act as a resonator at saidpredetermined frequency when said structure is coupled to said signalgenerator,

said length dimension being related to (2n1))\/ 4 1 1 wherein n is aninteger and A is the wave length of said acoustic signals, and eachchamber upon becoming a resonator having pressure standing wavesestablished therein to allow said generator to transmit into said mediumsaid powerful acoustic signals. 13. The combination as defined in claim12 wherein said length dimension is substantially equal to 14. Thecombination as defined in claim 12 wherein said signal generator iselectrically driven.

15. The combination as defined in claim 12 wherein said fluid medium isair.

16. The combination as defined in claim 15 wherein the chambers in saidstructure which transmit said acoustic signals into said medium aresuitably displaced from each other to allow said acoustic signals tobecome transmitted in a directional pattern.

17. The combination as defined in claim 16 and further including:

at least one sound reflector positioned adjacent to said structure toallow said acoustic signals to become reflected from said reflector.

References Cited UNITED STATES PATENTS 1,221,859 4/1917 Honold 340-3841,733,718 10/1929 Blondel 181-26 1,761,568 6/1930 Kersten 18127 XR2,087,052 7/1937 Spens Steuart 181,-.5 2,225,312 12/1940 Mason 18l.52,598,994 6/1952 Gougeon 340-3 88 2,720,934 10/1955 Schenkel l81.52,790,164 4/1957 Oberg 34038 8 3,046,544 7/1962 Auer et a1. 340-3883,138,795 6/1964 Wallace et a1. 340-384 3,153,783 10/1964 Whitehead340-384 3,214,753 10/1965 Dodge 340-384 FOREIGN PATENTS 6404318 10/ 1964Netherlands.

STEPHEN J. TOMSKY, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,517,390 June 23 1970 Layne Whitehead It is certified that error appears inthe above identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2 line 32 after "shown" insert in Column 3 line 15, "a shouldread an line 17, "efficiet" should read efficient line 41, "couplers."should read couplers line 43, "couplers." should read couplers; Column4, line 29, "intial" should read initial Column 5, line 27, after"multiple" insert a comma; line 66, "than" should read then Column 6,line 15, cancel "to"; line 46, "an" should read a line 48, after "each"insert individual coupler. The transformation ratio of each line 69, "a"should read a comma. Column 8 line 4 "of" should read off line 59 after"6" insert a comma.

Signed and sealed this 16th day of February 1971.

[SEAL] Attest:

EDWARD M.FLETCHER,JR. WILLIAM E SCHUYLER, JR. Attesting OfficerCommissioner of Patents

